1. Field of the Invention
The present invention relates to an optical repeater used in a transmission system and, more particularly, to an optical repeater having a loop-back function used in a submarine optical cable communication system.
2. Description of the Related Art
In a transmission system between two or more terminal stations using repeaters, a transmission of service signals and a monitoring of the whole transmission system are usually important, and a loop-back method (loop-back operation), which is one monitoring method and is used to check a faulty portion from the terminal station through the repeater, are known in the prior art. Further, it is preferable to apply the loop-back method to the transmission system to detect and confirm a faulty portion thereof.
For example, in a submarine optical cable communication system using optical marine cables, a plurality of optical repeaters are provided at specific intervals (from about several tens of kilometers to about one hundred kilometers) in the optical marine cable, to amplify optical signals. This submarine optical cable communication system is used to communicate between two or more landing stations at a distance of about from a few hundred kilometers to ten thousand kilometers, and is used to realize a long distance communication, e.g., an international communication. Further, in the submarine optical cable communication system, the loop-back method is used to detect and confirm a faulty portion in the optical marine cables or the optical repeaters which may be located under the sea (submerged) at a depth of 8,000 meters.
In the prior art optical repeater, for example, optical signals are converted to electrical signals by opto-electrical conversion units of an up transmission line and a down transmission line. Further, electrical signals are carried out in various processes, e.g., an equivalent amplification process, an identificative regeneration process, and a monitor-control process, by signal processing units, and then the processed signals are reconverted to optical signals by electro-optical conversion units of the up transmission line and the down transmission line. In a transmission system or submarine optical cable communication system of the prior art, indirect optical repeaters are mainly used.
In the above described optical repeater of the prior art, or the indirect optical repeater, conversion processes between the optical signals and electrical signals must be carried out, and thus a circuit scale of the repeater becomes large and the power consumption thereof is increased. Further, when a loop-back operation is carried out, the loop-back signals are mixed, at an optical mixer, with optical signals of the opposite transmission line output from the other terminal station.